1. Field of the Invention
The present invention relates to airboat propulsion mechanisms and, more particularly, to gear-based transmissions for airboats.
2. Description of Related Art
Airboats are often driven over land and water at high speeds. Airboats typically have employed aircraft engines operating at approximately 2500-3000 revolutions per minute (rpm) connected to solid direct-drive shafts, which rotate a single propeller. Aircraft engines are extremely expensive, and it is a general practice therefore to mount a used aircraft engine to an airboat to save on costs.
The steering apparatus of an airboat usually comprises a pair of rudders, with trim tabs added to correct for the torque that results from the rotation of the propeller, this torque tending to keep the boat from maintaining a level attitude.
Extreme gyroscopic forces can occur when airboats are turned rapidly, and these forces are borne, among other structures, by the driven shaft.
Previously known airboat systems utilize belt-driven transmissions, which are inefficient owing to power losses caused by belt friction, especially at higher rotational velocities. Belt breakage in these systems is a source of failure. Another disadvantage of belt-driven systems is their inability to permit reduction of propeller speed, since the driven shaft used to effect such a reduction would have to be too small to be practicable. Thus it would be advantageous to utilize a different transmission method in an airboat to enable engine speed reduction without loss of efficiency.
Propeller breakage is also a major source of failure, since at 3000 rpm extremely high forces are experienced at the propeller hub. It would therefore be desirable to reduce the load on the propeller.
It has been taught by Becker et al. (U.S. Pat. No. 4,932,280, dated Jun. 12, 1990) to use coaxial drive shaft systems for driving multiple outputs from a single input in an aircraft. Gearing means are disclosed for driving two outputs at different speeds.
The use of a gear-based transmission for airboats has been taught by Kaye (U.S. Pat. No. 5,807,149), including a transmission for driving a pair of counter-rotating coaxial shafts, to each of which is affixed a propeller. Such an arrangement can be used with an automobile engine, which is far more economical than an aircraft engine. This transmission has been shown to reduce noise and torque, to permit varying gear ratios, to increase fuel efficiency and engine life, and to be less expensive to operate.
An improved gear-based transmission for airboats has also been disclosed by Jordan (U.S. Pat. No. 5,724,867, the entire disclosure of which is incorporated herein by reference).